Photolithography is commonly used during formation of integrated circuits on semiconductor wafers. More specifically, a form of radiant energy (such as, for example, ultraviolet light) is passed through a radiation-patterning tool and onto a semiconductor wafer. The radiation-patterning tool can be, for example, a photomask or a reticle, with the term “photomask” traditionally being understood to refer to masks which define a pattern for an entirety of a wafer, and the term “reticle” traditionally being understood to refer to a patterning tool which defines a pattern for only a portion of a wafer. However, the terms “photomask” (or more generally “mask”) and “reticle” are frequently used interchangeably so that either term can refer to a radiation-patterning tool that encompasses either a portion or an entirety of a wafer.
Radiation-patterning tools contain light restrictive regions (for example, totally opaque or attenuated/half-toned regions) and light transmissive regions (for example, totally transparent regions) formed in a desired pattern. A grating pattern, for example, can be used to define parallel-spaced conductive lines on a semiconductor wafer. The wafer to be patterned is provided with a layer of photosensitive resist material commonly referred to as photoresist. Radiation passes through the radiation-patterning tool onto the layer of photoresist and transfers the mask pattern to the photoresist. The photoresist is then developed to remove either the exposed portions of photoresist for a positive photoresist or the unexposed portions of the photoresist for a negative photoresist. The remaining patterned photoresist can then be used as a mask on the wafer during a subsequent semiconductor fabrication step, such as, for example, ion implanting or etching relative to materials on the wafer proximate the photoresist.
In patterning photoresist, the patterned radiation emitted from the reticle passes through optics comprising one or more lenses and then onto the photoresist. As any one of the individual lens receives impinging radiation, it will begin to heat. If this radiation is directed to only certain points on the lens (either symmetrically or asymmetrically) and the thermal conductivity of the lens material is too low to distribute the heat load uniformly across the lens within the timeframe of the radiation exposure, then the lens will distort due to thermal expansion of the lens material in more heated regions relative to less heated regions. This results in optical aberrations when the distortion becomes significant. One manner of compensating for this is to impinge radiation more uniformly across the lens so that the heat load is distributed uniformly within the timeframe of the radiation exposure.
However, needs remain for improved methods of mitigating asymmetric lens heating in photolithographically patterning a photoresist using a reticle.